An ore is a naturally occurring rock or sediment that contains valuable minerals, typically metals, which can be extracted at a profit. The process of obtaining metals from the earth transforms this raw rock into a pure, usable material. The field dedicated to this transformation is called metallurgy, which encompasses extracting metals and preparing them for practical use. This process moves from initial physical preparation through chemical extraction and finally to rigorous purification to achieve the desired commercial grade metal.
Preparing and Concentrating the Ore
Preparing the raw ore for chemical processing involves removing most of the unwanted rock and sediment, known as gangue. This preliminary work begins with comminution, which involves crushing large, mined ore chunks into smaller pieces, followed by grinding them into a fine powder. Reducing the particle size increases the surface area of the valuable mineral, maximizing the efficiency of subsequent separation and chemical reactions.
Once the ore is finely ground, concentration methods physically separate the metal-bearing mineral from the gangue. Flotation is a common technique, primarily used for sulfide ores, where the powdered ore is mixed with water, oil, and frothing agents. Air is blown through the mixture to create a froth, and the valuable mineral particles selectively adhere to the oil-coated bubbles and float to the surface, while the gangue settles at the bottom.
Gravity separation, sometimes called hydraulic washing, utilizes the difference in density. A stream of water washes away lighter gangue particles, allowing the denser metal-containing particles to settle. Magnetic separation is used when either the ore or the gangue possesses magnetic properties, such as with iron ores like magnetite, allowing them to be separated by passing the crushed material over a magnetic roller.
Extracting the Metal
After concentration, the metal compound must be chemically reduced to yield the pure metal element. The choice of extraction method depends on the metal’s chemical properties and its reactivity.
Pyrometallurgy
Pyrometallurgy utilizes high temperatures to extract metals. Smelting is the most common pyrometallurgical process, involving heating a concentrated metal oxide or sulfide ore with a reduction agent like carbon or coke. This agent chemically removes the oxygen from the metal compound, leaving behind the molten metal, and is widely used for metals like iron and copper.
In a blast furnace used for iron production, carbon monoxide gas acts as the primary reducing agent, removing oxygen from iron oxides at high temperatures. A flux material, such as limestone, is added to react with remaining impurities to form slag. This molten waste layer floats on the surface of the liquid metal and is easily removed.
Hydrometallurgy
Hydrometallurgy involves using aqueous solutions to selectively dissolve the metal from the ore (leaching). This method is often preferred for low-grade ores. The powdered ore is contacted with a lixiviant, which is a chemical solvent that dissolves the target metal compound.
A well-known example is using a cyanide solution to leach gold from its ore. For copper, acid leaching with sulfuric acid is frequently used to dissolve copper from oxide ores. The metal ions are then recovered from the resulting “pregnant” solution through processes like electrowinning or precipitation.
Electrometallurgy
Electrometallurgy employs electrical energy to reduce a metal compound. This process is carried out in an electrolytic cell, where an electric current is passed through a molten salt or aqueous electrolyte containing the metal ions. The positively charged metal ions migrate to the negatively charged cathode, where they gain electrons and are deposited as pure metal.
The Hall-Héroult process for aluminum production is an example where aluminum oxide is dissolved in molten cryolite and reduced by a powerful electric current. Electrometallurgy can be used as a primary extraction method for metals above aluminum in the electrochemical series or as a final purification step.
Purification of Extracted Metals
The metal produced directly from the extraction stage, often called crude or blister metal, still contains impurities that can negatively affect its properties. Refining is the final stage, necessary to achieve the high purity levels required for commercial applications.
Fire Refining
Fire refining involves melting the crude metal, and impurities are removed through selective oxidation. Oxygen or air is blown through the molten metal, causing impurities to oxidize before the metal itself. The oxidized impurities are then skimmed off as a slag or evolve as a volatile gas. This technique is used for metals like copper, iron, and lead.
Electrorefining
For the highest purity requirements, electrorefining is the standard method. This process uses an electrolytic cell where the impure metal acts as the anode, a thin sheet of pure metal acts as the cathode, and an electrolyte solution contains the metal ions. As a direct current is applied, pure metal ions dissolve from the anode and are deposited onto the cathode. Electrorefining is commonly used to produce high-purity copper and to recover precious metals like gold and silver present in the crude metal.